~을/ㄹ걸 (~eul/~ri-eul-geo) – might be/most probably
This is essentially the same thing as ~(은/ㄴ & 을/ㄹ)것 같다 and you would attach this particle directly to a verb.
The main difference is you can’t use ~을/ㄹ걸요 for nouns – use ~인 것 같다 for nouns instead.
Past tense rules 았/었/였/했을걸요 apply.
그 남학생은 모든 시험 결과가 너무 좋아서 진짜 열심히 공부했을걸요.
All of that male student’s examination results are really good so he had studied really hard most probably.
우리 여기 공항에서 30분정도로 기다려서 지금쯤 비행기가 도착했을걸요.
We waited for 30 minutes at this airport here so the plane might have arrived by now.
아까 점심 시간에 많이 먹어서 지금 배가 아직 너무 불러 죽겠으니 저녁을 못 먹을걸요.
During lunch time just now, I ate so much that my stomach is still really bloated to death so I most probably can’t eat dinner.
오늘은 친구와 약속이 있는데 지금 비가 많이 와서 못 만날걸요.
I have an appointment with my friend today but the rain is very heavy now so I most probably can’t meet him.
How are these different from -을/ㄹ수도 있다 and -을/ㄹ지도 모르다?
Sorry, your explanation ‘The main difference is you can’t use ~을/ㄹ걸요 for nouns’ is totally wrong.
예) 그 사람은 학생일걸요 (학생이다+을걸요)
그 사람은 학생인 것 같아요 (학생이다+은 것 같다)